Magnetic recording hard disk drives use read and write transducers or heads mounted on a head carrier for reading and writing data to the disk. The head carrier is typically a gas-bearing slider attached to an actuator arm by a suspension and positioned very close to the disk surface by the suspension. There is typically a stack of disks in the disk drive with a slider-suspension assembly associated with each disk surface in the stack.
The separation or spacing between the slider and the disk surface is called the fly height. The slider has a disk-facing gas-bearing surface (GBS) that causes the slider to ride on a cushion or bearing of gas, typically air or helium, generated by rotation of the disk. The slider is attached to a flexure on the suspension and the suspension includes a load beam that applies a load force to the slider to counteract the gas-bearing force while permitting the slider to “pitch” and “roll”. The flying dynamics of the slider and thus the fly height are influenced by factors such as the rotation speed of the disk, the aerodynamic shape of the slider's GBS, the load force applied to the slider by the suspension, and the pitch and roll torques applied to the slider by the suspension.
Disk drives with thermal fly-height control (TFC) of the read and write heads have been proposed. One type of TFC uses an electrically-resistive heater located on the slider near the write head. When current is applied to the heater, the heater expands and causes the write head's write pole to expand and thus move closer to the disk surface. The write pole can be adjusted to different heights, depending on whether the drive is reading or writing. Also, the heater can maintain the write pole at the optimal fly-height even in the presence of the above-described factors, which would otherwise cause changes in the fly-height. The TFC heater must be accurately calibrated so that the head-disk spacing can be controlled. The calibration requires that the value of heater power that results in onset of head-disk contact or “touchdown” be known. This touchdown is intentional and may be performed on each drive during initial drive calibration, or periodically during normal operation. An embedded contact sensor (ECS), embedded in the slider near the write head, can be used to sense touchdown. The ECS comprises a metallic strip located at the slider GBS. The resistance of the ECS changes in response to temperature changes so the voltage across the ECS can be used to determine touchdown, when the slider temperature changes as it comes in close proximity to the disk.
TFC heater calibration using touchdown is required for conventional perpendicular magnetic recording (PMR) disk drives as well as for proposed heat-assisted magnetic recording (HAMR) disk drives and microwave-assisted magnetic recording (MAMR) disk drives. In a typical HAMR write head, light from a laser diode is coupled to a waveguide that guides the light to a near-field transducer (NFT) (also known as a plasmonic antenna). A “near-field” transducer refers to “near-field optics”, wherein the passage of light is through an element with subwavelength features and the light is coupled to a second element, such as a substrate like a magnetic recording layer, located a subwavelength distance from the first element. The slider supports the NFT and the write pole, with the NFT and write pole having ends located at the surface of the slider that faces the recording layer. A protective slider overcoat is formed on the recording-layer-facing surface over the NFT and write pole ends and serves as the GBS.